Mechanical processing is an important step in the manufacture of engineering components and is used not only to achieve the required shape but also to impart desirable changes in the microstructure and properties. In recent years, however, science-based methodologies are being developed for net shape or near net shape processing with the aim of optimizing the process parameters such that the mictostructure of the component may be controlled, and defects and flow instabilities are avoided on a repeatable basis in a manufacturing environment. A new method is Modeling of Dynamic Material Behavior which accounts for the dynamic metallurgical processes during hot deformation. Another criteria for predicting metallurgical instabilities in processing is instability map. In this criteria, instability is evaluated by parameter ζ which goes negative when there is a metallurgical instability. From the combination of these two criteria we can decide optimum processing parameters. In this article, purpose is to find optimum processing parameters of Ti-6Zr-6Nb-6Sn alloy using Modeling of Dynamic Material Behavior.
High temperature compression test of Ti-6Zr-6Nb-6Sn alloy was accomplished by using Thermecmaster at various strain rates(0.001, 0.01, 0.1, 0.5, 1, 10) and temperatures(650, 700, 750, 800, 850, 900℃). Generally, continuous flow softening behavior is observed in high strain rate but oscillatorty mode is exhibited in low strain rate. Dynamic material behavior during hot deformation has been studied by using a model of Prasad et al. The result shows that the microstructure, deformed in the region of high efficiency, is consisted of recrystallized α and β. Dynamic recrystallization is observed to be primary dynamic process occuring at high efficiency. A microstructural instability region has been also determined. Prasad et al's model is generally good agreement with the result experimental observation. Unstable microstructure is associated with shear band which is perpendicular to the compression axes. From these results, optimum processing condition is determined to be the high efficiency and microstructural stable region which shows highly recrystallized microstructure.
From the result of this article, the results shows that the highest efficiency, maximum material flow, region is under the condition of 0.001 strain rate and 750 ℃ temperature in 0.5 strain power dissipation map but this region shows negative value in parameter ζ. Therefore, this region was not considered as optimum processing condition. Also the microstructure of this region shows, beside of recrystallized α and prior β grain, shear bands which is consisted of widmannst tten α and prior β grain elongted in the way perpendicular to compression axis. This microstructure explains unstable flow properties of that rigion clearly in agreement with instability map. The optimum processing region of Ti-6Zr-6Nb-6Sn alloy is under the condition of 10 strain rate and 750℃ temperature. This region shows high efficiency(61%) and stable flow properties. Also completely recrystallized α and prior β are observed in microstructure. Such a high processing efficiency and microstructural stable flow propperties are explained by dynamic recrystallization which is a primary metallurgical process during hot deformation.
